An auto thermal reforming reactor is divided into two parts consisting of a combustion chamber followed by a catalyst bed. The geometry of the combustion zone and burner has a direct effect on the degree of mixing between the process gas and oxygen. In this study, using computational fluid dynamics (CFD), the effect of different burners on the combustion reaction in the combustion chamber has been investigated.The results showed that the optimal location of the burner is before the bottleneck, Because in this case, by increasing the gas speed during the combustion process, the length of the flame becomes longer and the output temperature increases.